1. Field of the Invention
This invention relates to a method for the projection welding of high-carbon steels, and high-tension low-alloy steels.
2. Description of the Prior Art
Projection welding is a method of lap resistance welding like spot or seam welding. A high current and a high pressure are concentrated on the projections formed on one or both of two parts to be joined, so that the heat generated by their contact and specific resistances may melt the materials and join the parts together, as is well known. Corners, edges, ends, bulged portions, etc. on the parts are sometimes utilized without any such projection being formed.
For projection welding, it is necessary that a movable electrode be so low in inertia and friction as to be capable of following the decay of the projections precisely, and that a uniform pressure be applicable by the whole electrode to enable uniform multi-spot welding. Projection welding requires a rigid welding machine and an accurately and quickly responding mechanism for applying pressure. Thus, there is a welding machine designed exclusively for projection welding.
Projection welding undesirably requires a better welding machine of higher performance and the preparation of projections with a considerably high dimensional accuracy, but also has many merits as stated below:
(1) It is useful even for the joining of parts differing from each other in thickness and therefore in heat capacity, since the projections formed on the part having the larger thickness make it easy to obtain a thermal equilibrium;
(2) It is useful even for the joining of different kinds of metals, since the projections formed on the metal superior in thermal conductivity make it easy to obtain a thermal equilibrium;
(3) The electrode having a large surface area is beneficial for mechanical strength and thermal conductivity, and is consumed only slowly;
(4) The uniform application of current and pressure to all of the spots to be welded gives substantially uniformly welded spots of high reliability;
(5) The simultaneous welding of a multiplicity of spots ensures a very quick and efficient job;
(6) The use of a special electrode or jig enables the accurate welding of parts complicated in shape; and
(7) It is useful for the joining of a wide range of materials, including steel, bronze, stainless steel, a nickel or aluminum alloy, and a combination of steel and brass or bronze.
Despite its numerous merits stated above, however, no projection welding has been applicable to the joining of two parts of high-carbon structural steel having a high hardenability, or of a part of high-carbon steel and another of high-tension low-alloy steel. Although projection welding is useful for joining different kinds of metals, or a wide range of materials as stated, it is applicable only to materials having a low carbon content and not showing any welding defect, such as cracking, and is hardly applicable to high-carbon structural steels of high hardenability, or high-tension low-alloy steels, such as S45C, SCM, SCNM or HT780. No sound welded joint free from any welding defect can be obtained on any such high-carbon, or high-tension low-alloy steel, since carbon promotes cracking or an increase of hardness as a result of rapid heating and cooling by which resistance welding is characterized. Thus, there has not been any method of projection welding used successfully in joining S45C or like high-carbon, and high-tension low-alloy steels.
Under these circumstances, it is an object of this invention to provide an improved method of projection welding which enables the sound welding of high-carbon, and high-tension low-alloy steels by an existing projection welding machine.
This object is essentially attained by a method for the projection welding of two parts of which at least one is of high-carbon, or high-tension low-alloy steel, wherein a spacer is disposed between those parts. The high-carbon steel may be any structural steel of high hardenability, such as S45C, SCM, SCNM or HT780, and the spacer may be in the form of a thin sheet having a thickness of 50 microns to 0.4 mm, or a coating having a thickness of 10 to 100 microns. If the spacer is a coating, it may be formed on at least one of the parts to be welded, or a combination of a thin sheet and a coating can alternatively be formed on one of the parts. The spacer may be of low- or ultralow-carbon steel having a carbon content of 0.05% or less, or pure nickel or copper, and the coating may be of iron, nickel or copper. The welding may be carried out in a non-oxidizing or reducing gas atmosphere, or in a vacuum.
The spacer is intended for diluting the carbon in the parts to be welded, and thereby avoiding any cracking, or increase of hardness caused by carbon. When the projections are gradually decayed by an electric current to form nuggets, the spacer is also melted into the nuggets and its material dilutes the carbon in the nuggets. The spacer, which is a thin sheet, or coating, forms a thin soft layer in the center of each joint.
The spacer preferably has a thickness of 50 microns to 0.4 mm if it is in the form of a thin sheet. A sheet having a thickness smaller than 50 microns is too expensive to be easily available on the market and is not easy to handle, either. A sheet having a thickness over 0.4 mm forms a joint layer having a substantially equal thickness irrespective of its own thickness if pressure is applied under equal conditions, and if its thickness is too large, extra metal protrudes from the joint and gives it a poor shape. The spacer in the form of a coating preferably has a thickness of 10 to 100 microns. A coating having a thickness smaller than 10 microns may be useless, as it peels off in an instant if too high a pressure, or current is applied thereto. A coating having a thickness over 100 microns may be of low quality and fail to form a joint of high quality.
The spacer is preferably of low- or ultralow-carbon steel having a carbon content of 0.05% or less, or pure nickel or copper, so that it may not form any hard and brittle intermetallic compound, but may form in the center of a joint a soft and ductile layer which will act as a buffer to prevent any reduction in notch fatigue or static strength even if the joint may be so poor in shape as to form a notch.
The method of this invention is preferably carried out in a nonoxidizing or reducing gas atmosphere, or in a vacuum to ensure the formation of a joint of high quality, since the exposure of the joint to the air at a high temperature is likely to result in the oxidation of its outer or inner surface, the formation of pores, or its lowering in quality by absorbing oxygen from the air. It may also be effective to apply an electric current to the welded joint again to lower its hardness and improve its elongation and toughness to a further extent by resistance heating.
Other features and advantages of this invention will become apparent from the following description and the accompanying drawings.